1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display apparatus. More particularly, the present invention relates to a backlight unit for illuminating a liquid crystal or the like from the rear side thereof, and a liquid crystal display apparatus comprising the backlight unit
2. Description of the Related Art
In general, a liquid crystal display apparatus comprises a liquid crystal panel comprising a pair of opposed substrates, between which a liquid crystal layer is sandwiched. An electrode is provided on each of the pair of opposed substrates. By applying a display voltage to the liquid crystal layer via the electrodes to change the orientations of liquid crystal molecules within the liquid crystal layer, light incident on the liquid crystal layer is transmitted/blocked so as to display characters, graphics or the like on the display screen.
A backlight unit is provided at the rear side of the liquid crystal panel. The backlight unit comprises a light source and a light guide plate. Light is input from the light source to an end side of the light guide plate. The input light is propagated through the light guide plate and is output through a broader side (proximal to the liquid crystal panel) of the light guide plate. The backlight unit allows light output by the light guide plate to impinge on the liquid crystal panel. There is a particular demand for a larger-sized, thinner, and brighter liquid crystal display apparatus.
Hereinafter, the configuration of a liquid crystal display apparatus comprising a conventional backlight unit will be described.
FIG. 10 is an exploded perspective view of a conventional liquid crystal display apparatus 300, showing the configuration of the major parts thereof.
Referring to FIG. 10, the liquid crystal display apparatus 300 comprises a liquid crystal panel 101 and a backlight unit 110 provided at the rear side of the liquid crystal panel 101.
The backlight unit 110 comprises a frame 111 for housing other members. The frame 111 houses a reflection sheet 112, alight guide plate 113, and optical sheets 114 and 115 in this order from the bottom.
The reflection sheet 112 is provided so to reflect light emitted from the rear side of the light guide plate 113 and causes the light to propagate toward the front of the light guide plate 113. The optical sheets 114 and 115 are provided so as to make uniform the light which is emitted from the front side of the light guide plate 113 and illuminates the liquid crystal panel 101.
A light source (lamp) 116 comprising an LED, a fluorescent tube, or the like, is provided along an end of the light guide plate 113. The light source (lamp) 116 is inserted into the frame 111 from the rear side thereof. A lamp cover 117 is attached to the frame 111 so as to surround the lamp 116. The lamp cover 117 is provided so as to protect the lamp 116 from damage and reflect light from the lamp 116 toward the light guide plate 113. A reflective thin film is provided on a surface of the lamp cover 117 by attaching, coating, depositing, or the like.
Circuit boards 102 and 103 each comprising a peripheral drive circuit are attached and connected to the liquid crystal panel 101. The liquid crystal panel 101 is mounted on the backlight unit 110. The circuit boards 102 and 103 are folded onto the rear side of the backlight unit 110. Board covers 104 and 105 are provided so as to insulate and protect the circuit boards 102 and 103. In addition, a front bezel 120 is attached to the front side of the liquid crystal panel 101.
The liquid crystal display apparatus 300 is assembled as follows.
The reflection sheet 112, the light guide plate 113, and the optical sheets 114 and 115 are stacked and housed in the frame 111 in this order from the bottom.
Next, the light source (lamp) 116 comprising an LED, a fluorescent tube, or the like is inserted into the frame 111 from the rear thereof. Thereafter, the lamp cover 117 is attached to the frame 111. Thus, the assembly of the backlight unit 110 is completed.
Next, the liquid crystal panel 101 is mounted on the backlight unit 110. The circuit boards 102 and 103 are folded onto the rear side of the backlight unit 110. The board covers 104 and 105 are attached to the backlight unit 110, respectively. The front bezel 120 is attached from above to the liquid crystal panel 101 so far obtained. Thus, the assembly of the liquid crystal display apparatus 300 is completed.
FIG. 11 is an exploded perspective view of another conventional liquid crystal display apparatus 400, showing the configuration of the major parts thereof.
Referring to FIG. 11, in the liquid crystal display apparatus 400, a reflection sheet 112, a light guide plate 113, and optical sheets 114 and 115, are stacked and housed in a frame 111 of a backlight unit 110 in this order from the bottom, as in the liquid crystal display apparatus 300 of FIG. 10.
A light source (lamp) 116 comprising an LED, a fluorescent tube, or the like is provided along an end of the light guide plate 113. The light source 116 is inserted into the frame 111 from the rear side thereof. A reflector 118 is previously attached to a portion of the frame 111 which partially surrounds the lamp 116. The reflector 118 reflects light from the lamp 116 toward an end of the light guide plate 113.
Circuit boards 102 and 103 each comprising a peripheral drive circuit are provided around the liquid crystal panel 101, as in the liquid crystal display apparatus 300 of FIG. 10. The liquid crystal panel 101 is mounted on the backlight unit 110. The circuit boards 102 and 103 are folded onto the rear side of the backlight unit 110. A back bezel 121 is attached to the rear side of the liquid crystal panel 101 so as to enhance the strength of the backlight unit 110, insulate and protect the circuit boards 102 and 103, and prevent the breakage of the lamp 116. In addition, a front bezel 120 is attached to the front side of the liquid crystal panel 101.
The liquid crystal display apparatus 400 is assembled as follows.
The reflection sheet 112, the light guide plate 113, and the optical sheets 114 and 115 are stacked and housed into the frame 111 in this order from the bottom.
Next, the light source (lamp) 116 comprising an LED, a fluorescent tube, or the like is inserted into the frame 111, to which the reflector 118 is attached, from the rear side thereof. Thus, the assembly of the backlight unit 110 is completed.
Next, the liquid crystal panel 101 is mounted onto the backlight unit 110. The circuit boards 102 and 103 are folded onto the rear side of the backlight unit 110. The back bezel 121 is attached to the rear side of the backlight unit 110. The front bezel 120 is attached from above to the liquid crystal panel 101 so far obtained. Thus, the assembly of the liquid crystal display apparatus 400 is completed.
In recent years, there is a demand for a lower-cost, lower-weight, and thinner liquid crystal display apparatus. With the advance of manufacturing technology, separate parts for different purposes, such as the board covers 104 and 105, the lamp cover 117, and the like, are attached to the rear side of the backlight unit 110 as in the liquid crystal display apparatus 300 of FIG. 10 rather than the liquid crystal display apparatus 400 of FIG. 11.
FIG. 12 is a perspective view of a conventional light guide plate 113, showing the structure of the major parts thereof.
Referring to FIG. 12, the conventional light guide plate 113 comprises ears 119 for positioning and prevention of displacement when the light guide plate 113 is housed in the frame 111. However, no device is provided so as to enhance the strength of the light guide plate 113.
Japanese Laid-Open Publication No. 9-282919 discloses a technique for solving the poor strength of the light guide plate.
Conventionally, the light guide plate is separated from the frame since if the light guide plate and the frame are joined together, luminance irregularity occurs at the joint. In the conventional case, however, the strength of the backlight unit is poor. Japanese Laid-Open Publication No. 9-282919 discloses a technique to solve this problem as follows. Plate-like flange portions are provided, which protrude from either the front side or the rear side of a light guide plate and have a thickness of substantially the half of the thickness of the light guide plate. Flange receiving portions corresponding to the flange portions are formed on a side of the frame which faces the flange portions with a gap therebetween. The flange portions and the flange receiving portions are welded together with spot-like welding portions. Therefore, the light guide plate and the frame are integrated together while keeping the gap therebetween, so that the strength of the backlight unit is improved.
Japanese Laid-Open Publication No. 2001-75490 discloses a liquid crystal panel holder which is provided between a liquid crystal panel and a lamp in order to reduce the number of parts, and a liquid crystal display apparatus comprising the liquid crystal panel holder. The liquid crystal panel holder has a diffusion plate for diffusing light from the lamp and a lamp holder for the lamp. The diffusion plate and the lamp holder are integrally molded of a resin.
Japanese Laid-Open Publication No. 2002-109931 discloses a backlight unit comprising a light guide plate in which light received from the incident side is reflected on the reflection side and is emitted through the light emitting side. In order to reduce the thickness of the backlight unit, a metal reflection film is deposited on the reflection side of the light guide plate and minute pits and projections are provided on the light emitting side opposed to the reflection side so as to diffuse light.
The conventional backlight unit 110 raises the following three problems.
(1) The strength of the backlight unit 110 is insufficient and the warping and flexing of the backlight unit 110 are not sufficiently prevented.
The strength of the backlight unit 110 is mainly improved by combining the frame 111 with the light guide plate 113 to enhance the rigidity of the backlight unit 110, so that the warping/flexing of the backlight unit 110 is minimized.
Further, the back bezel 121 is provided in addition to the frame 111 and the light guide plate 113 so as to improve the rigidity of the backlight unit 110, so that the warping/flexing of the backlight unit 110 is minimized.
Japanese Laid-Open Publication No. 9-282919 discloses that the strength of a backlight unit is improved by welding the flange portions of a light guide plate with the flange receiving portions of a frame with spot-like welding portions into an integral form.
(2) It is not easy to assemble the backlight unit 110.
The backlight unit 110 is generally assembled as follows. The reflection sheet 112, the light guide plate 113, and the optical sheets 114 and 115 are housed in the frame 111 in this order. Next, the light source (lamp) 116 is inserted into the frame 111. The lamp cover 117 is optionally attached to the frame 111.
Recently, frames 111 for housing other members are becoming smaller and thinner. Therefore, the frame 111 itself is less rigid, and warps and flexes under its own weight. Therefore, it is difficult to house members in the frame 111. It is necessary to delicately handle the assembly of the backlight unit in order to avoid the displacement of members from the frame 111, assembly errors, and the like. As a result, the assembly time is elongated, leading to an increase in the manufacturing cost of the backlight unit.
In recent years, there is a demand for a lower-cost liquid crystal display apparatus. A large cost cutting is also required for the backlight unit 110. Therefore, a reduction in the number of members, the integration of members, the elimination of a member, simplification of assembly, and the like have been tried to reduce cost.
For example, Japanese Laid-Open Publication No. 2001-75490 discloses that the number of members is reduced by integrating a diffusion plate with a lamp holder so that the assembly is made easier. However, in this publication, a backlight is provided right under a liquid crystal panel without a light guide plate. Such a backlight system (direct backlight system) is fundamentally different from a backlight unit comprising a light guide plate in terms of structure.
(3) It is not easy to produce a smaller-sized and thinner backlight unit 110 having a narrower-picture-frame.
Conventionally, the size of the backlight unit 110, the size of the picture-frame for the screen of a liquid crystal display apparatus, and the thickness of a liquid crystal display apparatus, are determined by the frame 111 for housing the light guide plate 113. Therefore, no liquid crystal display apparatus having a picture-frame size smaller than the size of the frame 111 and a thickness smaller than that of the frame 111 has been achieved.
Japanese Laid-Open Publication No. 2002-109931 discloses that a metal reflection film is deposited on the reflection side of a light guide plate and minute pits and projections are provided on the light emitting side of the light guide plate so that the thickness of the backlight unit is smaller than when the same functions are otherwise achieved by using other members.